The present invention relates to a variable flow resistance calibrator which is used for calibrating the flow indicator of a high volume air sampler.
A high volume air sampler is a conventional apparatus for determining the concentration of total suspended particulate in the ambient atmosphere. The apparatus generally comprises a filter, a blower or motor driven fan, and a rotometer. The rotometer is preferably an inexpensive device including an air column and known weight ball. A calibrator is used with this apparatus so that a plot can be established associated with the apparatus correlating the observed flow meter reading with the true air flow in cubic feet per minute.
The rotometer, as previously mentioned, may simply be a tube having a ball of known weight therein with the height that the ball is raised being a function of the air flow. Typically, a small portion of the total volume is sampled by a tube leading to the rotometer. It is a standard technique to use a relatively inexpensive rotometer and calibrate the rotometer readings to provide true air flow.
The high volume air sampler is usually operated over a predetermined period of time such as, for example, a twenty-four hour period. An initial flow reading is taken and also a final flow reading. From these two readings, an average air flow can be calculated. The filter is also weighed at the beginning and end of the test period. The weight that is collected in the filter is the difference between the initial and final weights. With these values a weight concentration can easily be calculated giving a value of weight of particulate that is collected per total air flow.
The prior art calibrator is provided with a series of resistance plates. It is quite common to use five resistance plates, each of which must be sequentially installed and removed from the calibrator. Their installation and removal requires the unscrewing of the large knurled ring at the base of the instrument, the insertion of the resistance plate, the careful positioning of two gaskets usually, and a rethreading of the knurled ring. Thus, with the prior art device this is a time consuming and inconvenient process.
Accordingly, one object of the present invention is to provide an improved flow resistance calibrator that obviates the prior art problem of using a number of different size resistant plates. In accordance with the present invention only two plates are employed, one of which is fixed and the other one of which is rotatable relative to the fixed plate to provide different air flow values.
A further object of the present invention is to provide an improved variable flow resistance calibrator wherein the flow can be set to virtually any value by a fine adjustment between the plates.
Still a further object of the present invention is to provide a variable flow resistance calibrator that is relatively simple in construction, can be made relatively inexpensively, is durable, and can be operated quite simply.